Limit account volume

Since almost all forms of intervention operate on a logged-in user, a common strategy for scaled abuse is the creation of many accounts, either managed by a single user switching between contexts, or using automated tools. Before getting too much into this, it's important to note that there are some legitimate cases where a user would want (or need) access to multiple accounts. Politicians, kids followed by their parents, LGBTQ youth in unsupportive situations, people who do social media as a job, in each case, the person's ability intentionally cultivate their identity requires the use of distinct personae. Platforms tend to understand and support this need, but their support drops off quickly - most platforms are fine with users having two accounts - most don't want users to have more than ten.

Today, platforms that want to enforce their limits on account creation tightly do so through SMS verification, in combination with limiting the number of accounts that can be tied to a given phone number - they might allow a user to use the same phone number for two or three accounts, but block the creation of the fourth account with the same phone number. 

Platforms that are less concerned about the problem, but still don't want automated access to their systems, often gate the creation of an account on the solving of a captcha.

Just like everything, there isn't a "right" answer here. A platform should set up explicit expectations of how many accounts they expect a user to have in the most permissive use cases, and then find technical ways of enforcing that boundary. This is essential to get right before a platform scales, since intervention at the account creation stage is infinitely easier than trying to unwind a complex network of personae after they've been created and used.